This invention relates to a vertical aperture correction circuit and, more particularly, to such a circuit which can be used in a color image pickup device to emphasize changes in the brightness level of the video picture from one line interval to the next while minimizing any line-crawling effect which might be inherent in the video signal.
In the color television picture which is derived from many color television cameras, the so-called edge sharpness, or sensitivity, of that picture is not as well-defined as in the picture derived from black-and-white television cameras. That is, a transition in brightness, or contrast, from one horizontal line interval to the next may not exhibit a desirable level of sharpness. Consequently, a viewer may not perceive accurate detail in the vertical direction. This loss of sharpness in the vertical direction, that is, in the direction perpendicular to the direction of line scanning, is analogous to aperture aberrations in an optical system.
Various proposals have been suggested for improving this sharpness. Such compensation or correction systems have been referred to generally as vertical aperture compensation systems. In one type of vertical aperture compensation system, the luminance signal, which may be the television signal generated by a black-and-white television camera or the luminance component of a composite color television signal generated by a color television camera, is delayed by one horizontal scanning, or line, interval, and the difference between the delayed and undelayed luminance signals then is derived. If the luminance level in successive line intervals is approximately the same, the aforementioned difference signal exhibits a relatively low amplitude. However, when the brightness level changes from one line interval to the next, this difference signal will be more pronounced. Consequently, the difference signal can be used as a relatively accurate indication of brightness changes in the vertical direction.
To emphasize such brightness level changes in the vertical direction, that is, to obtain vertical aperture compensation, a predetermined proportion of the difference signal is added to the original, i.e. undelayed, luminance signal. The summed signal thus is a vertical aperture-corrected luminance signal.
The aforementioned vertical aperture-correction technique is accompanied by undesired interference when used in a Trinicon color television camera. In the Trinicon camera, the target end of the pickup tube is provided with a set of index electrodes. These index electrodes are supplied with an index signal whose polarity is reversed at each horizontal scanning interval, thereby superimposing an alternating index signal onto the photoelectroconductive target. This index signal appears as a periodic fluctuating voltage level superimposed onto the luminance signal derived from the Trinicon camera. When the aforementioned vertical aperture-compensation technique is used with this luminance signal, the superimposed periodic, fluctuating level is emphasized. Therefore, in addition to providing an indication of brightness level changes from one line interval to the next, the vertical aperture-compensated luminance signal is provided with an emphasized, superimposed periodic fluctuating level which results in a line-crawling effect in the video picture ultimately reproduced therefrom.
In addition to this periodic fluctuating level derived from the index signal, another AC component may be introduced into the luminance signal due to the operation of the typical DC-DC converter that is used with the television camera. This DC-DC converter is provided in the television camera for the purpose of generating various DC control voltages from a single supplied DC voltage. Generally, during normal operation thereof, the DC-DC converter is supplied with relatively large amounts of power, and this has been known to introduce an AC component into the relatively low-level luminance signal. Such an AC component may appear as noise, typically a striped pattern, on the reproduced television picture. To minimize this noise, the driving frequency of the DC-DC converter may be synchronized to one-half the horizontal scanning frequency. However, this is the very same frequency of the index signal that results in a superimposed periodic, fluctuating level on the luminance signal. Thus, when the vertical aperture-compensation technique discussed above is used, the AC component derived from the DC-DC converter is manifested in the aforementioned line-crawling effect.
One technique which has been proposed for eliminating the line-crawling effect due to the superimposed index signal and, presumably, also will eliminate the line-crawling effect due to the AC component derived from the DC-DC converter, is disclosed in U.S. Pat. No. 4,160,265. According to this patent, the difference between the delayed and undelayed luminance signal, which is indicative of line-to-line brightness-level changes and which also emphasizes the periodic fluctuations superimposed onto the luminance signal, is squared, or multiplied by itself, and the squared difference signal then is mixed with the sum of the delayed and undelayed video signals. The output of the mixing circuit is a vertical aperture-corrected luminance signal that is substantially free of undesired periodic level fluctuations which may be due to the superimposed index signal of the Trinicon camera or may be due to the AC component produced from the DC-DC converter.
In accordance with the present invention, vertical aperture-correction is attained in the absence of any line-crawling effect, and the complexity of the correction circuit due to the aforementioned squaring circuit is reduced.